Experimental and numerical studies on the NO reaction pathways of NH3 cofiring with coal volatile matter

Ammonia (NH3) cofiring provides a promising solution to carbon reduction of coal-fired boilers, but it may lead to increased NOx emissions. However, many experiments observed that the NOx emissions exhibited an increase-then-decrease trend with the increase of NH3 cofiring ratio (RNH3), indicating that NOx could be controlled under high RNH3. To reveal the underlying mechanism, the experiments of NH3 cofiring with coal volatile were first conducted in a one-dimensional flow reactor to reveal that this trend is primarily attributed to the gaseous reactions of volatile and NH3. A chemical reactor network model was then constructed to investigate the influences of RNH3 on the NO reaction pathways of NH3. The predicted results replicated the increase and then decrease trend of NOx emissions as RNH3 increased beyond 25 %. It was found that NO formation is primarily controlled by the reactions between HNO/N and OH/H radicals. Under lower RNH3, the concentrations of HNO/N increase with the increase of RNH3 leading to increased NO formation. Under higher RNH3, however, much of the OH/H radicals are consumed by the dehydrogenation reactions of NH3 which consequently inhibits the NO formation reactions. Therefore, the root mechanism of the increase-then-decrease trend of NOx emissions is attributed to the competition for the OH/H radicals between the initial and final steps of the NO formation reaction pathway.